Hydraulic fracturing system

ABSTRACT

A method is given for fracturing a formation, in particular far-field in a tight formation, in which at least a portion of the proppant is crushable in situ at some point during pumping, during fracture closure, or at higher stresses experienced later during fracture closure. The closure stress or hydrostatic stress is estimated, then a proppant is selected that is at least partially crushable at that closure stress, and then the fracturing treatment is performed with at least a portion of the total proppant being the selected crushable proppant.

BACKGROUND OF THE INVENTION

Hydraulic fracturing is an effective method of increasing hydrocarbonproduction. The method involves pumping of a fracturing fluid into asubterranean formation (i.e. reservoir) through a wellbore under apressure exceeding the formation stress. A propping material is placedin the resulting fractures to prevent them from closing, which, thus,provides unimpeded flow paths and enhanced transport of hydrocarbonsfrom the reservoir to the wellbore.

The art of hydraulic fracturing is based to a great extent on materials:the fluids with their various constituents and the proppants withoptional auxiliary particulates. The proppant materials are intended toprovide enhanced hydraulic conductivity of a fracture under theformation closure stress. The traditional approach to proppant design isfocused on several material characteristics, which include: a)compressive strength or crush resistance under formation closure stress,to avoid generation of fines, which are known to damage proppant packconductivity; b) low specific gravity, to place the proppant deep into afracture with a fluid of reasonable viscosity; c) substantiallyspherical proppant particulate shape with smooth particle surface anduniform size distribution to maximize proppant pack permeability; and d)low material cost. These parameters are contradictory, so there isusually a trade-off between the properties. As an example, proppantcrush resistance, which is a characteristic of material mechanicalstrength, often conflicts with the required proppant low density and lowcost. The choice of proppant also strongly depends on the properties ofthe targeted reservoir, which can vary significantly. Therefore, whileproppant pack conductivity is often considered as a primary proppantcharacteristic, in certain cases, it can be sacrificed to achieve otherbenefits. In very tight reservoirs, even very low fracture conductivitywill still result in a suitable flow path for hydrocarbons entering fromthe formation.

It would be desirable to have an inexpensive proppant that is readilytransported deep into fractured formations by low viscosity fluids andneed not have low specific gravity, particle size or shape uniformity,or strength.

SUMMARY OF THE INVENTION

One embodiment of the invention is a method of hydraulic fracturing asubterranean formation penetrated by a wellbore including the steps of(a) estimating the closure stress in a fracture, (b) selecting acrushable proppant that produces more than about 20 percent fines in acrush test using that closure stress, and (c) injecting a slurry of theproppant in a carrier fluid into the formation. The crushable proppantmay be, for example, in the form of spheres, plates, disks, rods,cylinders, platelets, flakes, sheets, scales, husks, chips, shells,lumps and mixtures thereof. The crushable proppant may be a mixture ofparticles of at least two different shapes that have at least twodifferent crush strengths. The crushable proppant may include particlesof at least two different materials that have at least two differentcrush strengths. The crushable proppant may be entirely or partiallyceramic hollow spheres, glass or ceramic microspheres and microballoons,cenospheres, plerospheres and combinations of those materials. Thecrushable proppant may be partially or entirely made of materials withclosed porosity, such as glass and ceramics, rocks and minerals,polymers and plastics, metals and alloys, composite materials,biomaterials and combinations of those materials. The materials withclosed porosity may have fibrous, arch/cellular, mesh, mesh/cellular,honeycomb, bubble, sponge-like or foam structures and combinations ofthese structures. The crushable proppant may be made of finer materialthat has been formed into larger particles by agglomeration or binding.The crushable proppant may be coated. The crushable proppant may be usedat a concentration of from about 10 to about 100% of the total solids inthe slurry. The crushable proppant should produce more than about 10percent, preferably more than about 15 percent, fines in a crush testusing the closure stress of the formation.

In other embodiments of the invention, step (c) is followed by injectionof a slurry in which the proppant is not crushable. A cycle ofalternating proppant types may be repeated a plurality of times. Thenon-crushable proppant should generate less than about 6 to about 20percent fines in a crush test using the closure stress of the formation,for example as delineated in API RP56 for various mesh sizes ofproppant. Optionally, a portion of the crushable proppant may be crushedduring step (c) and/or a portion of the crushable proppant may becrushed when the fracture closes after step (c).

In other embodiments, the formation may have a permeability of less thanabout 0.001 mD and the proppant loading may be less than about 4.88kg/m². The proppant optionally includes at least 10 weight percent ofmica or cenospheres or mixtures of those materials. Optionally, theproppant may be continuously added to a carrier fluid injected into theformation. The crush strength of the material may be chosen so that atleast a portion of the crush occurs after initial cleanup of the well.The surface treating pressures may be reduced relative to injectingconventional proppant at similar proppant concentrations. The settlingvelocity may be less than that of 150 micron sand.

Yet another embodiment is a method of hydraulic fracturing asubterranean formation penetrated by a wellbore including the steps of(a) estimating the hydraulic pressure to which materials will be exposedduring pumping, (b) selecting a crushable proppant that produces morethan about 20 percent fines in a crush test using that hydraulicpressure, and (c) injecting a slurry of the proppant in a carrier fluidinto the formation. The hydrostatic pressure may be changed during thestep of injecting to control crushing of the crushable proppantmaterial, for example the rate of injection may be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows spherical proppant particles having a diameter just smallerthan a tube.

FIG. 2 shows packs of plate-like particles in a fracture.

FIG. 3 shows experimental proppant pack conductivities of muscovite micaat various proppant loadings and various closure pressures.

FIG. 4 shows the dependence of mica pack conductivity on proppantloading at various closure stresses.

FIG. 5 shows experimental proppant pack conductivities of muscovite micaMD250 at a proppant loading of 0.49 kg/m² at various closure pressuresand various flow rates.

FIG. 6 shows conductivity data for cenospheres and mica at a proppantloading of 0.49 kg/m² under various closure stresses.

FIG. 7 compares experimental settling velocities of conventionalfracturing sands and mica flakes.

DETAILED DESCRIPTION OF THE INVENTION

Although the following discussion emphasizes fracturing far into tightformations, the proppants and methods of the Invention may be used inany fracturing setting. The invention will be described in terms oftreatment of vertical wells, but is equally applicable to wells of anyorientation. The invention will be described for hydrocarbon productionwells (gas, oil, condensate), but it is to be understood that theinvention may be used for wells producing other fluids, such as water orcarbon dioxide, or, for example, for injection or storage wells. Itshould also be understood that throughout this specification, when aconcentration or amount range is described as being useful, or suitable,or the like, it is intended that any and every concentration or amountwithin the range, including the end points, is to be considered ashaving been stated. Furthermore, each numerical value should be readonce as modified by the term “about” (unless already expressly somodified) and then read again as not to be so modified unless otherwisestated in context. For example, “a range of from 1 to 10” is to be readas indicating each and every possible number along the continuum betweenabout 1 and about 10. In other words, when a certain range is expressed,even if only a few specific data points are explicitly identified orreferred to within the range, or even when no data points are referredto within the range, it is to be understood that the inventorsappreciate and understand that any and all data points within the rangeare to be considered to have been specified, and that the inventors havepossession of the entire range and all points within the range.

We have found methods of hydraulic fracturing utilizing crushableparticulates, which provide sufficient and cost effective fractureconductivity insignificantly dependent on closure stress. Furthermore,such particulates can also be used to deliver proppant far-field (deepinto the reservoir away from the wellbore) into a complex fracturenetwork, where no high strength proppants can be placed via currentpractices. Potential applications of the crushable particulates includeunconventional reservoirs, for example tight gas shales, because therequired proppant conductivity in such reservoirs can be relatively lowand the proppant transport properties become much more important. (Theterm proppant is used here to refer to materials with sufficientcompressive strength to hold a fracture open.) In one embodiment of theinvention, crush of the spherical or non-spherical particulates leads torelative increase of effective porosity in the pack, due to opening offlow channels. The closure stress impact on the proppant packconductivity is significantly diminished, as with pack compression anddecrease of the total pack porosity, the fraction of effective porosityincreases. In another embodiment of the invention, hollow and/or highlyporous lightweight particulates are delivered deep into a fracture,wherein they are crushed with a formation closure stress, and stillprovide sufficient conductivity to the fracture, as it is propped withfragments of the initial particulates.

All proppants are crushable at some closure stress, so proppants mayinadvertently have been used above their crush strengths in the past.This has always been considered deleterious to treatment effectivenessand success, but we have now discovered that under certain circumstancesit may be advantageous to estimate the closure stress that will beexperienced in a proposed fracturing treatment and then to select aproppant that will be substantially crushed under those circumstancesNot all proppants are suitable as the crushable proppants of theinvention; they should have certain properties as injected, such as slowsettling, and certain properties when crushed. For example, the basematerial may have sufficient strength to support the formation stresses,but may have defects in the structure and/or in the mechanical design orstructure of the material that results in weak points that will breakunder formation closure stress. Generation of fines upon crushing whenusing large proppant sizes can be a problem, as fines restrict theconductivity of large-proppant packs. In the present invention, however,particles are selected to crush during pumping to create small particlesthat transport effectively or during fracture closure to create adequateconductivity. Relevant properties include size and shape; mixtures ofdifferent sizes and/or shapes can be used. Also, crush resistance forsome of the shapes should be less than the available stress. When usinga mixture of shapes, some shapes should have substantially differentcrush strengths and may exceed the available stress. For example, ahollow sphere proppant material that crushes under hydrostatic pressurecan be used to create small, strong particles that transport effectivelyand support formation closure stresses.

It is widely accepted that proppant crush is highly undesirable, as itleads to formation of fines, which fill pores of the proppant pack, thusdecreasing its hydraulic conductivity. Significant efforts have beenmade in the past to develop high strength proppants (HSP) and many suchproducts are available commercially. Most HSP's are based on ceramicmaterials and are characterized by relatively high cost and highspecific gravity.

Patent Application PCT/RU2008/000566 discloses plate-like materials,which, while they possess quite low conductivity, compensate withsignificantly enhanced transport properties, e.g. slower settlingcompared to sand of similar density and particle size. The reason forsuch slowed settling is so-called glided settling, as the plate-likeparticle settling path length is significantly increased. Whileconductivity of mica in packs is quite low, it can be deliveredfar-field with a fluid of low viscosity without settling issues, whichdeleteriously affect spherical particulates of similar size. Note thatthe cost of mica can be significantly lower than that of polymericmaterials. Other benefits of the plate-like proppants include diminishedproppant embedment into formation fracture faces, due to the greatercontact surface area of proppant particles with the formation, andenhanced flowback control. Preferred plate-like proppants are layeredrocks and minerals; most preferred is mica, for example muscovite mica.Some of these materials may be crushable and suitable for use in thepresent Invention.

We define a plate-like particle as one which possesses three averagedimensions in which the largest dimension is at least two times thesmallest dimension and the third dimension can be smaller or equal tothe largest dimension. Thus, discs with a thickness of less thanone-half the diameter qualify as plate-like particles. Rod-like andfiber-like particles can also be used. Preferably, such particles shouldhave an aspect ratio of at least about 2, most preferably at least about3; preferably they should have a maximum length of about 5 mm, mostpreferably about 3 mm. Such particles can be made, for example, of glassor ceramics or may be of natural origin, for example basalt fibers,asbestos fibers and the like.

Another trend in proppant development targets lightweight and ultralightweight particulates. These proppants are intended for use inunconventional reservoirs (i.e. gas shales, tight gas sands), whereslickwater is used as a fracturing fluid. Slickwater is typically adilute solution of a friction reducer (a polymer is added to decreasepumping pressure), the viscosity of which generally does not exceedabout 10 mPa-s. Slickwater treatments are pumped in large volumes in gasshales to create complex fracture networks, which are believed toenhance gas production. As delivery of a proppant into the fracturenetwork with a fluid of low viscosity is challenging, the conventionalapproach to proppant development for slickwater treatments is to reducethe proppant specific gravity. Lightweight proppants based on polymercomposites have been commercialized; they commonly have specificgravities of from about 1.08 to about 1.25 g/cm³. The main problem withthese proppants is their cost, but they are technically suitable for thepresent Invention.

Therefore, other suitable proppants are described in U.S. Pat. No.4,547,468 which discloses hollow fine-grained ceramic proppants, thathave a crushing strength equal to or greater than that of Ottawa sand atclosure stresses above 5000 psi (34.5 MPa). Also suitable, U.S. Pat. No.7,220,454 and U.S. Patent Application Publication 20070154268 describehigh strength polycrystalline ceramic spheres and methods of makinghollow spheres of alumina or aluminate by coating of polymeric beadswith alumoxane, heating the particles to convert alumoxane to alumina,removal of the polymeric beads from inside the coating by washing with asolvent, and sintering the resulted hollow particles to give highstrength α-alumina spheres. Also suitable as proppants in the Inventionare those described in U.S. Patent Application Publication Nos.20070154268, 20070166541, 20070202318, and 20080135245; these discloseproppants having a suitable crush resistance and/or buoyancy as shown byspecific gravity. These proppants generally are made with a templatematerial and a shell on the template material; the shell is a ceramicmaterial or oxide thereof or a metal oxide. The template material may bea hollow sphere and may be a single particle, such as a cenosphere.

The concept of the present Invention differs from the aforementionedpatents and patent applications, and others describing hollowlightweight proppants, by utilization of the hollow particle as atransport vehicle for the true proppant only. The proppant material inthe present Invention is not the particles themselves, but rather theirfragments. Such an approach completely changes the design of theproppant material, as there is no need to reinforce the particulates,but rather they may be crushable, and chosen to provide the bestpossible conductivity of the crushed material.

The stress distribution in a pack of spherical particles is usuallyquite uniform; however, above critical compressive stresses theparticles start crushing. While proppant particle crushing haspreviously been believed to be bad for the pack permeability, this isnot necessarily true. One can consider a simple notional experiment (seeExample 1 below) which demonstrates that proppant crush can actuallyincrease permeability under certain conditions. If the fracturing slurrycontains particulates which are subject to crushing at the closurestresses, such a crush, however, may induce generation of channels (flowpaths) in the pack, and hence enhances permeability, which is theninsignificantly affected by closure stress magnitude. The crushableparticulates may be approximately spherical or in the form of plates,disks, platelets, flakes, sheets, cylinders, rods, scales, husks, chips,shells and mixtures thereof (see Examples 2 and 3 below). If notspherical, the crushable particulates may have any aspect ratio.

Alternatively, the fracturing slurry may contain crushable particulatesof low specific gravity. Such particulate material may be chosen from avariety of lightweight materials, including, but not limited to, ceramichollow spheres, glass microspheres and microballoons, cenospheres,plerospheres (char and ash cenospheres which have their cavities filledby finer particles of ash and other materials), various porousmaterials, including rocks and minerals, ceramics, cements, polymers;and various composite materials and mixtures of such materials. Thepreferred material is cenospheres, hollow spherical ceramic particlesmade of aluminum and silica, which are a byproduct of coal combustionand are found in fly ash. The particles are filled with air and haveapparent specific gravities of from about 0.4 to about 0.8 g/cm³. Theirprimary use is as fillers for cements to make low density concrete (seeExample 4 below). However, placement of such lightweight particulatesdeep into a fracture or complex fracture network may be easily achievedby means of fluids of low viscosity, i.e. slickwater, because theparticles are generally buoyant. When the fracture is closed, theparticulates are subjected to crush, which generates particle fragments,which still prop the remote fractures open, thus providing permeabilitysufficient for hydrocarbon production enhancement.

Alternatively, the crushable proppants of the Invention may beparticulates made of materials with closed porosity, for example glassand ceramics, rocks and minerals, polymers and plastics, metals andalloys, composite materials, biomaterials and combinations of suchmaterials. Such closed porosity materials may have fibrous,arch/cellular, mesh, mesh/cellular, honeycomb, bubble, sponge-like orfoam structures and combinations of such structures. Any proppant undersufficient closure pressure is a crushable proppant.

Alternatively, the crushable proppants of the Invention may be crushedinto fragments due to hydrostatic pressures encountered during pumpinginto the reservoir. In this case, fine mesh proppant materials arecreated in situ during pumping. An example is fine mesh materials thatare delivered to the location in a granulated/pelletized form. Crushingof the aggregates during pumping or fracture closure reduces dusting andother HSE risks encountered at the surface. Cenospheres and otherfragile particulates fall into this category; at least a portion oftheir crushing may occur during pumping, not necessarily under formationclosure stress.

Alternatively, the proppant may include a mixture of plate-like or, asother examples, rod-like or cylinder-like and either approximatelyspherical particles or irregular particles so that the plates or, asother examples, rods or cylinders will trap the spheres or irregularparticles in between layers. This increases the permeability of the packof plates or, as other examples, rod-like or cylinder-like materials.Then, under stress, the spheres may be a failure point (if they arelower strength than the plates) or an initiation point for the plates tocrack (if they have higher crush strength than the plates). Before anycracking of plates occurs, the permeability should be higher with thespheres.

It is preferred that crushing of the crushable proppants of theInvention produces particulates which are less than an order ofmagnitude smaller than the parent particles. The size distribution ofthe crushed material may be determined by experiments such as the API RP56 test described below.

All or a portion of the crushable particles may be coated to increasetheir strength, alter their wettability, provide higher closed porosityand thus better transport properties, reduce fines formation, decreasethe friction during pumping or decrease their adhesion to each other.Suitable materials for enhancing the properties may include quaternaryhydrophobic or hydrophilic absorbents, adsorbed surfactants, silicones,fluorocarbons, or polymers which impart desirable surface properties tothe particles. As another example, crushable proppant can be coated withresin coating that would provide higher crush resistance, therebyproviding higher conductivity during the initial flowback of the wellwhere stresses are lower to enhance fluid cleanup, and then the resincoated particles crush at higher stresses during production to create afine mesh pack. Further, the particle faces can be etched by chemical oroptical methods to make the surfaces rough rather than smooth to enhancepermeability.

The proppants and methods of the Invention are particularly suitable tovery tight formations, which, as used herein, refers to formationshaving a permeability less than about 1 millidarcy, and in variousembodiments, less than about 100 microdarcy, less than about 10microdarcy, less than about 1 microdarcy, or less than about 500nanodarcy. These formations have such low permeability that the wellscan be effectively stimulated in one embodiment with an overall orprimary final fracture conductivity on the order of 0.3 to 30 mD-m (1 to100 mD-ft) and/or with secondary and/or tertiary fractures on the orderof 0.0003 to 3 mD-m (0.001 to 10 mD-ft), where secondary fractures areunderstood to refer to usually relatively smaller fractures in lengthand/or width, branching from the primary fractures, and tertiaryfractures refer to usually relatively smaller fractures in length and/orwidth, branching from the secondary fractures. As an example, thecrushable proppants of the Invention may be used for treating formationswith permeabilities less than 0.001 mD where the proppant loading isless than about 4.88 kg/m² (1 lb/ft²), preferably less than about 2kg/m² (0.5 lb/ft²). As another example, the crushable proppants of theInvention may be used for treating formations where the generatedfractures are not substantially lateral but may include a mixture ofinduced lateral and transverse flow paths; the proppant is transportedthroughout the network of induced flow paths. Crushable proppant can becontinually injected or slugs of larger particles can be used to promotetransport.

Because the effectiveness of the final proppant pack does not rely onthe porosity or permeability of the packed matrix of the proppant asinjected to impart flow conductivity to the fracture, the availabilityof the option to select a wider range of proppant materials can be anadvantage in embodiments of the present invention. For example, theproppant may have a range of mixed, variable diameters, shapes,strengths or other properties that yield a suitable proppant pack afterclosure and crushing of at least some of the proppant. If the proppantas injected is uniform in properties, at least some of it must becrushed; if the proppant is a mixture of different materials or of onematerial but a mixture of, for example, sizes and/or shapes, then atleast one of the different proppants must be at least partiallycrushable under closure conditions. The crushable particles make up fromabout 10 percent to about 100 percent of the total particles in thefluid, preferably from about 30 to about 100 percent. The preferredconcentration of crushable particles in the fluid is from about 0.1 toabout 1200 kg/m³ (10 ppa), most preferably from about 120 kg/m³ to about240 kg/m³ (0.1 to about 2 ppa). The other proppant material may, forexample, be conventional proppant materials, such as sands, ceramics,sintered bauxites, glass beads, minerals, polymers, plastics, naturallyoccurring and composite materials and combinations of these materials.

Optionally, conventional proppants may be used to fill that portion ofthe fracture nearer the wellbore where, because of the size and geometryof the fracture created, the advantages of the crushable proppants ofthe Invention may not be needed. (The nearer the wellbore, typically thewider and less complex the fracture.) This conventional proppant mayhave a crush strength above the closure pressure. This conventionalproppant material may, for example, be conventional proppant materials,such as sands, ceramics, sintered bauxites, glass beads, minerals,polymers, plastics, naturally occurring and composite materials andcombinations of these materials.

Any surface and downhole equipment, any pumping schedule, and anyfracturing or slickwater fluids, may be used with the crushableproppants and methods of the Invention, provided only that the crushableparticles are not substantially broken by the equipment before theyreach the final pack position. Optionally, the equipment/proppant can beengineered to break the proppant prior to placement—for example in thewellbore (in situ crushing). Any additives conventionally used infracturing or slickwater fluids may also be used. When applied with lowviscosity fluids that have difficulty in transporting conventionalproppants, such as slickwater, higher concentrations of the crushableproppant can be employed as that reduces the settling rate.Alternatively, the practice of alternating slugs of slickwater withoutproppant and slickwater with proppant can be eliminated and proppant canbe continuously added to the slickwater. This practice reduces the useof water which can have a significant impact on the economics andsimplifies the pumping operation. The amounts and concentrations oftotal proppant can also be the same as for treatments of similarformations with similar fluids without crushable proppants.

While the fundamental connection is believed to exist, no correlationshave yet been reported between material compressive strength andproppant crush resistance. The latter is usually estimated forconventional proppants according to the API RP 56 method in a specialcrush cell under different applied loadings. The fines generated aremeasured by means of sieve analysis and compared with the dataprescribed by the API RP. The easiest way to define the crushableparticulate proppants of the Invention is to define them as those whichdo not meet the API RP 56 crush resistance specifications. Note thattherefore we define “fines” as being any particles produced by crushingof the original proppant.

API RP 56 describes the crush test as being done in a press havingplatens that can be maintained parallel and apply a load; the celltypically has an inside diameter of 5.08 cm (2 inches) and a pistonlength of 8.89 cm (3.5 inches). Proppant of a given mesh size range isplaced in the cell at a concentration of 19.5 kg/m² (4 lb/ft²), whichfor the cell described is 40 g, and leveled by inserting the piston androtating it in one direction. A load is applied, taking 1 minute toreach the maximum, and held for 2 minutes. The load is released and theproppant removed and sieved in a sieve shaker for 10 minutes. Anyparticles smaller than the smallest mesh size loaded are consideredfines. Results are compared to the suggested specifications given inTable 1:

TABLE 1 Mesh Size Mesh Size Stress Stress Maximum (U.S.) (mm) (psi)(MPa) Fines  6/12 1.68-3.36 2000 13.8 20  8/16 1.19-2.38 2000 13.8 1812/20 0.84-1.68 3000 20.7 16 16/30 0.59-1.19 3000 20.2 14 20/400.42-0.84 4000 27.6 14 30/50 0.297-0.59  4000 27.6 10 40/70 0.21-0.425000 34.5 8  70/140 0.105-0.21  5000 34.5 6The present invention can be further understood from the followingexamples.

EXAMPLE 1

A tube with a fixed inner diameter is filled with fragile sphericalproppant particles having nearly the same diameter as the tube (see FIG.1A). In this case, before closure stress is applied, the tube will havealmost zero permeability; even though the total porosity of the packmight be quite high, the porosity available for fluid flow isnegligible. Once an external closure stress is applied, and the innerdiameter is reduced, the proppant particles start crushing (FIG. 1B),the total pack volume decreases and so does the total pack porosity.However, the proppant crushing leads to opening of pore space and therelative effective porosity (which is a fraction of the total porosity)increases, which results in an increase in the tube permeability.

EXAMPLE 2

For non-spherically shaped materials, for example plates, this effectcan be even more pronounced, as the conditions in this latter case aremuch closer to those encountered in the field, as compared to those inthe first example. Consider non-spherical particles, for example plates,in a fracture having parallel walls. Without any externally appliedstress, the particles are oriented randomly (except perhaps for someorientation induced by transport fluid flow) and a substantial fractionof the pack porosity is confined (see FIG. 2A). Once the stress isapplied, the particles tend to align with the walls and some of them arecrushed, opening flow channels (see FIG. 2B). The effect of closurestress on the permeability of the pack therefore may be quitecomplicated, and permeability and effective porosity generally canincrease with closure stress applied on the pack. A similar result canbe obtained with rods and/or cylinders.

Laboratory Experiments Materials

A commercial muscovite mica sample was obtained from Minelco SpecialtiesLimited, Derby, UK. It was designated MD250; the number in the coderepresents the approximate maximum flake diameter in microns. Thethickness of these mica particles was about 20 to 25 microns. Themanufacturer described the material as dry ground, highly delaminatedpotassium aluminum silicate Muscovite Mica flakes having a melting pointof about 1300° C., a specific gravity of about 2.8, a pH of about 9 as a10% slurry in water, and as being flexible, elastic, tough, and having ahigh aspect ratio. The MD250 material is 99.9% smaller than 250 microns,10-50% smaller than 125 microns, and 0-15% smaller than 63 microns.

Cenospheres were obtained from Sphere Services, Inc., Oak Ridge, Tenn.,USA; they are lightweight, inert, hollow spheres made of silica andalumina and filled with air and/or gases. Cenospheres are a naturallyoccurring by-product of the burning process at coal-fired power plants,and they have most of the same properties as manufactured hollow-sphereproducts. The size given by the manufacturer is 10 to 350 microns.

Standard Conductivity Apparatus

The conductivity apparatus consisted of a 90, 700 kg (100 ton) loadpress with automated hydraulic intensifiers and API conductivity cellshaving 64.5 cm² flow paths. The apparatus could attain a maximum closurestress of 138 MPa and a maximum temperature of 177° C. The temperatureof the conductivity cells was controlled by electrically heated platenscontacting the sides of the cell. Precision metering pumps were used topump brine through the cell during flowback and conductivitymeasurements. The pumps drew 2 wt % KCl brine from a 20 l flowbackreservoir. The brine was vacuum degassed and nitrogen sparged to preventthe introduction of metal oxides into the proppant pack. The brine waspumped through a silica saturation system prior to entering theconductivity cell. Rosemount pressure gauges (with upper limits of 690Pa, 62 kPa and 2 MPa) were used to measure the pressure drop across theconductivity cell. Digital linear variable displacement transducers ortelescope width gauges were used to measure the distances between thecores to monitor fracture widths. The conductivity apparatus wasautomated for controlling closure stress ramps, flow schedules andtemperature, as well as providing data acquisition and real-timeconductivity/permeability calculations.

Split Core Apparatus

A Formation Response Tester (FRT) Model 6100 obtained from ChandlerEngineering (Broken Arrow, Okla., USA) was used for split coreconductivity measurements. A proppant pack was placed between two metalsemi-cylindrical cores, which were inserted into a rubber sleeve, inplace of the traditional cylindrical rock core sample. The confiningpressure was applied to the sleeve with a manual hydraulic pump. Thefully automated core flow instrument allowed the operator to sequencevarious fluids (including acids) through a core sample. The system wasdesigned to handle acids and other corrosive fluids at temperatures upto 177° C. The core could be up to 3.81 cm in diameter and up to 17.1 cmlong. Operating pressure and temperature were limited to 41.4 MPa and177° C. The direction of flow was flexible; flows from top to bottom,across the core face, and bottom to top (system flush) could all bedone. The differential pressure was measured across the core sampleusing two Rosemount precision differential pressure sensors. During theexecution of a test, conductivity was measured using a 0 to 2.75 MPadifferential pressure transducer or a 0 to 41.4 MPa transducer,depending on the range and level of precision required.

EXAMPLE 3

The conductivities of muscovite mica MD250 packs at proppant loadings of2.44 and 9.77 kg/m² were measured with the standard conductivityapparatus, while special precautions were taken to avoid parasitic flowsdue to the low proppant pack permeabilities. (Compared to conventionalproppant packs, the packs of plate-like mica particles werecharacterized by very low conductivities and by non-conventional stressdistributions in the packs. Ohio sandstone cores are usually used forAPI conductivity tests; however, in our case flows through such coreswere possible, which would have strongly affected conductivity results.Other parasitic flows might have existed due to unusually high pressuredrops in the cells (up to 1.72 MPa (250 psi)), e.g. flows along theconductivity cell walls. Other challenges in conductivity measurementsof mica packs have been faced. Special precautions used included: a)utilization of aluminum cores instead of sandstone ones; b) sealing thecore edges with room temperature vulcanized rubber; c) sealing the cellwalls with a silicon vacuum grease.) The conductivities of the micapacks at proppant loadings of 0.49 kg/m² and below were measured bymeans of the split-core apparatus. The steady state conductivity datafor the mica packs are presented graphically in FIG. 3. Theconductivities of the packs depended only weakly on the proppantloadings, as shown in FIG. 4. In this range of proppant loadings theconductivity would have been expected to be proportional to the proppantloading. Formation of channels in the pack decreased this dependence, asis demonstrated in FIG. 4.

EXAMPLE 4

The conductivity of a 0.49 kg/m² mica pack was found to be stronglydependent on flow rates, as shown in FIG. 5. The formation of channelsin the mica packs was observed when the conductivity cell wasdisassembled. The higher the flow rate, the more channels could be seen.

EXAMPLE 5

The conductivity of 0.49 kg/m² cenospheres was measured and comparedwith that of mica. The cenospheres exhibited a drastic drop inconductivity at closure stresses above 14 MPa, due to particle failure,as shown in FIG. 6. However, the retained conductivity may be sufficientto enhance production from extremely low permeable unconventionalreservoirs, for example gas shales where conductivities as low as 1.4mD-cm (0.05 mD-ft) are acceptable in secondary and tertiary fractures.

EXAMPLE 6

Static proppant settling measurements were performed in slickwater,comprising tap water containing 0.05 wt. % of a polyacrylamide-basedfriction reducer. The fluid was placed into a 500 ml graduated cylinderwith a ruler fixed on its side. A portion of proppant slurry (proppantin slickwater, 1:1 by volume) was slowly introduced into the cylinderwith a spatula and the settling front was photographed at 1-2 secondintervals. The falling front path was calculated and the terminalsettling velocity was determined from the linear part of the curve pathvs. time. Three replicate measurements were made for each material ineach fluid, and the velocities were averaged. The settling velocitiesare shown in FIG. 7. Mica flakes demonstrated significantly slowersettling rates than the conventional silica sands that are widely usedas proppants. Cenospheres did not exhibit any settling at all, as theparticles were floating on the slickwater surface.

Having thus described our invention, we claim:
 1. A method of hydraulicfracturing a subterranean formation penetrated by a wellbore comprising(a) estimating the closure stress in a fracture, (b) selecting acrushable proppant that produces more than about 20 percent fines in acrush test using that closure stress, and (c) injecting a slurry of theproppant in a carrier fluid into the formation.
 2. The method of claim 1wherein the crushable proppant is in the form of spheres, plates, disks,rods, cylinders, platelets, flakes, sheets, scales, husks, chips,shells, lumps and mixtures thereof.
 3. The method of claim 1 wherein thecrushable proppant comprises particles of at least two different shapesthat have at least two different crush strengths.
 4. The method of claim1 wherein the crushable proppant comprises particles of at least twodifferent materials that have at least two different crush strengths. 5.The method of claim 1 wherein the crushable proppant is selected fromthe group consisting of ceramic hollow spheres, glass or ceramicmicrospheres and microballoons, ceno spheres, plerospheres andcombinations thereof.
 6. The method of claim 1 wherein the crushableproppant comprises materials with closed porosity.
 7. The method ofclaim 6 wherein the materials with closed porosity, are selected fromthe group consisting of glass and ceramics, rocks and minerals, polymersand plastics, metals and alloys, composite materials, biomaterials andcombinations thereof.
 8. The method of claim 6 wherein the materialswith closed porosity have fibrous, arch/cellular, mesh, mesh/cellular,honeycomb, bubble, sponge-like or foam structures and combinationsthereof.
 9. The method of claim 1 wherein the crushable proppantcomprises finer material that has been formed into larger particles byagglomeration or binding.
 10. The method claim 1 wherein the crushableproppant is coated.
 11. The method of claim 1 wherein the crushableproppant comprises from 10 to 100% of the total solids in the slurry.12. The method of claim 1 wherein the crushable proppant produces morethan 15 percent fines in a crush test using the closure stress of theformation.
 13. The method of claim 1 wherein the crushable proppantproduces more than 10 percent fines in a crush test using the closurestress of the formation.
 14. The method of claim 1 wherein step (c) isfollowed by injection of a slurry in which the proppant is notcrushable.
 15. The method of claim 14 where a cycle of alternatingproppant types is repeated a plurality of times.
 16. The method of claim14 wherein the crushable proppant generates less than about 6 to about20 percent fines in a crush test using the closure stress of theformation
 17. The method of claim 1 wherein a portion of the crushableproppant is crushed during step (c).
 18. The method of claim 1 wherein aportion of the crushable proppant is crushed when the fracture closesafter step (c).
 19. The method of claim 1 wherein the formation has apermeability of less than about 001 mD and the proppant loading is lessthan about 4.88 kg/m².
 20. The method of claim 1 wherein the proppantconsists of at least 10 weight percent of mica or cenospheres ormixtures thereof.
 21. The method of claim 1 wherein the proppant iscontinuously added to a carrier fluid injected into the formation. 22.The method of claim 1 wherein the crush strength of the material ischosen so that at least a portion of the crush occurs after initialcleanup of the well.
 23. The method of claim 1 wherein the surfacetreating pressures are reduced relative to injecting conventionalproppant at similar proppant concentrations.
 24. The method of claim 1wherein the settling velocity is less than that of 150 micron sand. 25.A method of hydraulic fracturing a subterranean formation penetrated bya wellbore comprising (a) estimating the hydraulic pressure to whichmaterials are exposed during pumping, (b) selecting a crushable proppantthat produces more than about 20 percent fines in a crush test usingthat hydraulic pressure, and (c) injecting a slurry of the proppant in acarrier fluid into the formation.
 26. The method of claim 25 wherein thehydrostatic pressure is changed during the step of injecting to controlcrushing of the crushable proppant material.
 27. The method of claim 26wherein the rate of injection is increased.